1. Field of the Invention
The present invention generally relates to systems for preserving information relating to semiconductor integrated circuits and, more particularly, systems for preserving information regarding the manufacturing history of semiconductor integrated circuits.
2. History of the Prior Art
Semiconductor integrated circuits are batch fabricated on wafers of semiconductor material generally five to eight inches in diameter. Each wafer typically has a two-dimensional array of die, each die having one integrated circuit. These wafers are processed in groups which are referred to as fabrication lots. Each fabrication lot is identified by a unique fabrication lot number. Each wafer within a given fabrication lot is identified by a unique wafer number. The wafer number and the fabrication lot number for a particular wafer are inscribed on that wafer in an area outside the array of die. Thus, during the initial fabrication process, every die may be uniquely identified by certain die-specific information such as its fabrication lot number, its wafer number, and its location within the array of die hereinafter referred to as its die position number.
The unique identification of the die, if available, can serve important functions in the analysis of non-production or developmental die (e.g., die characterization, reliability testing) and of regular production die (e.g., wafer sort yield analysis, the analysis of die returned by customers). For example, engineers frequently need to characterize the die by analyzing the effect of a processing variable on the electrical properties of the die. To do this, the engineer will typically split a lot into groups of wafers during fabrication, each group to be processed with the particular processing variable set to a different value. After the die are processed and individually packaged, electrical parameters of each die are tested, and the test results are saved along with the die's wafer number and die position number. With this data an engineer is able to compare the performance of the groups of wafers with one another, and thus determine the effect of the processing variable on die performance. This information also enables an engineer to analyze the electrical performance of the die as a function of its position on the wafer during fabrication.
The unique identification of a die serves a similar purpose in the reliability testing of the die. To test its reliability, the die may be subjected to conditions such as extreme temperatures or humidity for extended periods and then electrically tested. The unique identification of each die enables an engineer to analyze the test results and determine the reliability of the die as a function of processing parameters, and also as a function of its position on the wafer during fabrication.
With respect to regular production die, a die's unique identification, if available, may also serve an important function in the analysis of defective die returned by customers. The die from a particular lot may be sold to many different customers over a long period of time. When defective die from a particular fabrication lot are returned by a customer, an engineer will typically analyze the returned die to determine the cause of the malfunction. The unique identification of the die allows the engineer to determine the manufacturing history of the returned die, which may provide information which is helpful in this analysis. Also, if the engineer knows which fabrication lot the die came from, he may be able to avoid repetitive analysis on future customer returns from that same lot.
Currently, those skilled in the art view the retention of the unique identification of a die throughout the life of the die as a time-consuming, expensive, and error-prone process. In the course of normal production, a die's unique identification could be useful in yield enhancement analysis and in the analysis of customer returns. Presently, however, the wafer number and die position number are usually lost after the wafer is sawed into individual die, and thus not available for subsequent yield enhancement analysis. Also, after the wafer is sawed, a new assembly lot number is assigned to each lot and encoded onto each package into which the die is assembled. The fabrication lot number is retained only in the paper records for that assembly lot. Thus, an engineer faced with a die returned from a customer must search the assembly lot records to find the fabrication lot number. These records may be incomplete or may have been misplaced or disposed of after a certain period of time. Without the information from these records, an engineer will not be able to determine the die's fabrication lot number. Even if the records are available and complete, the wafer number and die position number cannot be determined.
In order to retain the unique identification of each die for non-production purposes such as die characterization or reliability testing, the wafers are sent to a non-production assembly area with special instructions to preserve the identification of each die. The die's identification is usually preserved by handwriting the information on each individual package into which the die are assembled. This process is not automated, and it is possible that the die will be mixed up and identified incorrectly. Because there is no way to be sure that the die are identified correctly, confidence in the die characterization or reliability testing is diminished greatly.
From the foregoing it can be seen that it would be quite desirable to ensure that the unique identification of each die is recorded correctly and retained throughout the life of the die. It may also be seen that it is a shortcoming and deficiency of the prior art that heretofore no system or method for accomplishing this has been developed.